Published on December 22, 2008
X-ray Production : X-ray Production Sharif Qatarneh Medical Physics Division Department of Radiation Oncology King Hussein Cancer Center, Jordan. X-rays : X-rays Frequencies: >3 x 1016 Hz. Wavelengths: <10 nm. Quantum energies: >124 eV. X-ray production : X-ray production High energy electrons, accelerated by thousands of kilovolts of potential, interact with a metal target in an x-ray tube. X-ray production : X-ray production X-rays are produced when the electrons are suddenly decelerated upon collision with the metal target (brehmsstrahlung) or "braking radiation". If the bombarding electrons have sufficient energy, they can knock an electron out of an inner shell of the target metal atoms. Then electrons from higher states drop down to fill the vacancy, emitting x-ray photons with precise energies determined by the electron energy levels (characteristic x-rays). Bremsstrahlung X-rays : Bremsstrahlung X-rays Radiation which is emitted when electrons are decelerated or “braked” when they are fired at a metal target. Accelerated charges give off electromagnetic radiation When the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. Bremsstrahlung is characterized by a continuous distribution of radiation. Characteristic X-rays : Characteristic X-rays Electrons make transitions between lower atomic energy levels in heavy elements. They have definite energies since they have energies determined by the atomic energy levels. Vacancies are produced in the K-shell of the atom and electrons drop down to fill the gap. X-ray Generators : X-ray Generators Generator of power for x-ray tube. Control of energy and exposure. Tube voltage (kV) ? Energy. Tube current (mA) ? Radiation Quantity. Exposure time (s) ? Exposure. Transformers: Transforming the input voltage (Up or Down). Step-up transformer used in x-ray generators to increase voltage (up to 150 kV). Rectification: Changing Alternating Current (AC) voltage to Direct Current (DC) in the x-ray tube. X-ray Tube : X-ray Tube X-ray Tube - Cathode : X-ray Tube - Cathode Cathode filament is a coiled tungsten wire which is the source of electrons. Cathode focusing cup to direct electrons to target. The cathode acts to excite electrons to the point where they become free from their parent atom and are then able to become part of the electron beam. The cathode acts as a negative electrode and propels the free electrons, in the form of an electron beam, towards the positive electrode (the anode). X-ray Tube – Anode 1 : X-ray Tube – Anode 1 The anode acts as a positive electrode, attracting the free electrons and accelerating the electrons through the electromagnetic field that exists between the anode and cathode. This increases the velocity of the electrons, building potential energy. The higher the kV rating, the greater the speed at which the electrons are propelled through the gap between the cathode and anode. X-ray Tube – Anode 2 : X-ray Tube – Anode 2 The electrons then impact a target (e.g., tungsten, molybdenum). This causes the release of the potential energy built up by the acceleration of the electrons comprising the electron beam. Most of this energy is converted to heat and is radiated by the copper portions of the anode. The remainder is refracted off of the target in the form of high energy photons, or x-rays, forming the x-ray beam. Why rotating anode? X-ray Tube : X-ray Tube X-ray Tube - Glass envelope : X-ray Tube - Glass envelope Glass envelope: The above components are sealed into a glass envelope. This allows for gases and other impurities to be pumped out of the tube, creating the vacuum necessary for proper performance. X-ray Tube – Focal Spot : X-ray Tube – Focal Spot Focal spot is the apparent source of x-rays. Focal spot size is the dimension of x-ray source as viewed from the image. Focal spot size is affected by: Cathode filament shape. Focusing cup. Electric field between anode and cathode. Focal spot size affects x-ray images. Smaller focal spot size produce sharper image (better resolution). What is the Line Focus Principle?